Grey-scale ultrasound is a useful investigation in selected patients with a painful swollen leg. It is of particular value in cases in which there is a clinical suspicion of deep vein thrombosis (DVT) but other features, such as an atypical history or equivocal radiology, suggest alternative pathology. Five such cases are presented in which ultrasound showed transonic lesions. The cause of the swelling of the leg was thus shown to be "cystic' in nature and therefore not due to DVT. This enabled inappropriate and potentially harmful therapy to be avoided and the correct therapy, such as surgical drainage, to be undertaken.